August 19. Dimetrodon

If there was one animal that was iconic for the Permian, it would be the fin-backed Dimetrodon, discovered in 1878. When I was in college, dimetrodon was considered to be a reptile, and my historical geology textbook said “the reason for such extraordinary specialization is entirely problematical,” meaning of course, we didn’t know. But they were often called mammal-like reptiles, for some skeletal characteristics that appeared to put them closer to mammals than reptiles. 

Dimetrodon drawing by DiBgd at en.wikipedia used under GFDL CC-BY-2.5

Today dimetrodons and their relatives, pelecosaurs, a term that isn’t used much anymore, are considered to be synapsids, the group that includes mammals. I’ve seen synapsids given as a class of the tetrapods or four-limbed animals. We’re familiar with the other classes of tetrapods – amphibians, reptiles, birds, and mammals. Synapsids might be better considered to be a group that includes primitive ancestral forms that probably would not be classed as mammals today, together with modern mammals themselves.

Dimetrodons are classed as synapsids. Not reptiles, but not quite mammals either. In early Permian time they became the largest land vertebrates, up to 15 feet long. Their fossils have come almost entirely from Texas and Oklahoma, where they lived in lowland deltaic wetlands, but there are dimetrodons from Germany as well. There are at least 12 species of fin-backed dimetrodons. The spines on dimetrodons’ backs extend from their spinal vertebrae, making a sail-like fin as much as 10 feet high. The speculation that they used the sail for temperature regulation – warming the blood circulating through it, and radiating heat to cool the animal – dates back to 1940, but the question isn’t settled yet. Plenty of work has been done, but with little other than bones to go by, such a characteristic is really difficult to prove. An alternative explanation is that dimetrodons exhibited sexual dimorphism – males and females had consistently different body features – and the fin might have been related to some kind of mating display.

Dimetrodons are often thought of as early dinosaurs, but they were extinct at least 40 million years before the dinosaurs appeared. The didn’t even make it to the Great Dying at the end of the Permian; dimetrodons were extinct by the middle Permian, about 272 million years ago.

—Richard I. Gibson

Dimetrodon drawing by DiBgd at en.wikipedia used under GFDL CC-BY-2.5.

August 18. Permian reptiles

Bradysaurus, a Permian reptile

Reptiles were better suited to the dry, arid climates that were common during Permian time than amphibians, and reptiles proliferated and diversified during this time. The Pareiasaurs were large herbivores, two to nine feet long, with bony plates armoring their bodies. Reconstructions look rather like big horned toads. In some varieties the bony plates have grown together, suggesting that this group may be ancestors of modern turtles, but the group was extinct at the end of the Permian and it is not certain that they gave rise to descendents that became turtles. It may be that the coalescing bony plates, somewhat like turtles’ shells, might have developed independently in both lineages.  

Mesosaurs, of early Permian age, were among the first reptiles to return to the water. They were clearly adapted to an aquatic life, with webbed feet and a long, streamlined body. Its leg joints – wrists and ankles – were designed in a way that would have made it impossible for them to walk on land, but they might have waddled ashore to lay eggs as modern sea turtles do, but embryo fossils of mesosaurs are not associated with egg shells, so an alternative interpretation is that they bore their young alive. If so, they are among the first animals to do so.

A lot of the known Permian reptiles are lizard-like, several inches to a foot or so long, and many are presumed to have been insectivores, filling the ecological niche that similar reptiles do today. The basic body plan of these animals seems to have been well established by the Permian. One lizard-like Permian reptile called Eudibamus was described in 2000 from a fossil found in Germany. It is possibly the first bipedal reptile. And another, known from Madagascar, had a wide skin layer between its ribs that probably allowed it to glide like modern flying squirrels. All of these adaptations make it clear that the Permian was a time of experimentation and expansion for the reptiles.

* * *

On the night of August 17-18, 1959, the strongest earthquake recorded in the Northern Rocky Mountains struck the upper Madison River Valley near Hebgen Lake west of Yellowstone National Park. It measured about 7.4 on the moment magnitude scale and it triggered a huge landslide in Madison Canyon. The slide buried a campground, killing at least 26 people and damming the Madison River, which backed up to form a new lake, Quake Lake. The earthquake re-set the rhythms of geysers in Yellowstone Park and damaged buildings as far away as Butte and Bozeman. The fault scarps were as much as 19 feet high and can still be seen 50 years later. The faults were normal faults, dropping the Hebgen Lake basin down relative to the adjacent mountains.

—Richard I. Gibson

Drawing by Nobu Tamura (http://spinops.blogspot.com) used under GFDL

Reference:
Road Log for the Hebgen Lake Earthquake Area, by Michael Stickney, Tobacco Root Geological Society Guidebook (2012), p. 71

August 17. Glaciers and Coal

Today’s episode is a response to a listener’s question about the close juxtaposition of glacial deposits and coal beds in Australia.

Despite the abundance of coal in the Carboniferous, especially in the northern hemisphere, and despite the changing climate that meant coal formation there largely ended with the end of the Carboniferous, there’s plenty of Permian coal too. Most of it is in the former Gondwana – Australia, South Africa, India, South America, and Antarctica, but there is a lot of Permian coal in Russia as well. In Gondwana, the coal is pretty closely associated with glacial deposits.

We might expect that glacial deposits and coal swamps would reflect two very different environments, but so far as I can tell, they pretty much co-existed in Permian time at least in quite a few places.

In Australia and elsewhere, the coal-bearing rocks and glacially deposited layers actually interfinger. So at best, we might have had some relatively rapid changes in climate to switch from glacial times to warmer, coal-swamp times, and from what I read there were at least 8 specific glacial periods in the late Carboniferous and early Permian.

But the alternative explanation, and from what I can gather it seems to be the preferred one, is that these areas were on the margin of the ice, and plant life actually thrived there. The keys to making coal are 1) lots of plants and 2) rapid burial of the plant matter so it does not have time to decay. Our typical vision of warm swampy areas with low oxygen to prevent decay is just one way to do that. A cold climate, with plants buried by glacial debris, would work just as well, if not better.

Gondwana base map from Du Toit (1937, Our Wandering Continents). Blue line is highly generalized margin of glacial area; solid black are highly generalized coal deposits (based on Langford, 1992).

I think that while Australia was certainly part of the glaciation in the southern polar part of Pangaea, it was probably far enough away from the pole (which was more or less in South Africa, but there are coal deposits there, too) that the climate might not have been like modern Antarctica, but perhaps more like modern Patagonia but with glaciers. So abundant plant life could have been growing, even thriving, near the glacial margin. A modern analogy would be the peat bogs of temperate and even arctic climates. When glaciers receded, forests and peat on the tundra would advance. When glaciers advanced, the deposits the glaciers carried would have buried the forests. This would be a good way to get the interfingering of glacial and coal deposits that we do observe.

This would not strictly be cyclothems, which represent rises and falls of sea level, alternately allowing swamps to form and then burying them in river sediment, but they would be cyclic nonetheless, like cyclothems. I do not know if the coal-glacial sediment packages follow the 8 known glacial periods or are (likely) something more complex, but if they do it would be on a periodicity of a few million years. Standard cyclothems can show alternations that may represent changes on scales of a few tens of thousands of years, or even fewer, as well as the longer periods of millions of years. If the coal results from glacier-margin plant life, as I infer it does, then the alternations would not reflect sea-level changes as cyclothems do, but more directly would reflect changes in position of the glaciers (together with the dumping of sediment to bury the forests or other vegetation).

—Richard I. Gibson

Links:
Permian of Australia
Permian coal in South Africa 

Langford, 1992 - Gondwana’s Permian coal